This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 10-251124, filed Sep. 4, 1998, the entire contents of which are incorporated herein by reference.
This invention relates to a polymer electrolyte fuel cells system which makes use of a solid polymer as an electrolyte, and in particular to a polymer electrolyte fuel cells system having a mechanism for preventing the drying of a solid polymer electrolyte membrane.
A fuel cells system is designed such that a fuel gas such as hydrogen or a reactive gas is rendered to electrochemically react with an oxidizing gas such as air so as to directly convert the chemical energy of the fuel to an electric energy.
This fuel cells system can be classified into various types depending on the kind of electrolyte. As one type of fuel cells system, there is known a polymer electrolyte fuel cells system which makes use of a solid polymer as an electrolyte.
FIG. 1 shows a cross-sectional view of one example of the structure of such a polymer electrolyte fuel cells system.
Referring to FIG. 1, each cell 4 comprises a pair of gas-diffusing electrodes consisting of a fuel electrode (hereinafter, referred to as an anode) 1a and an oxidizing electrode (hereinafter, referred to as a cathode) 1b, and a polymer electrolyte membrane 3 having ion conductivity and gas-separating function which is interposed between the pair of gas-diffusing electrodes with a catalyst layer 2a or 2b (made of Pt for example) being interposed between the solid polymer electrolyte membrane 3 and each of the gas-diffusing electrodes. This cell 4 is capable of generating an electric output through a power generation by an electrochemical reaction between a fuel gas or a reactive gas and an oxidizing gas.
This fuel cells system is constituted by a plurality of cells 4 and a gas-impermeable separator 5 provided with grooves for feeding a reactive gas to each of the electrodes of the cells 4.
According to this fuel cells system, a fuel gas such as hydrogen is fed to the anode 1a, while an oxidizing gas such as air is fed to the cathode 1b so as to allow an electrochemical reaction to take place, thereby generating an electromotive force at each cell 4. This electromotive force of each cell 4 is as low as about 1V at most. Therefore, in order to obtain a high output, a cell stack comprising a laminate body of a plurality of cells 4 is put to practical use as a fuel cells system.
Since the electrochemical reaction in this fuel cells system is exothermic reaction, heat is caused to generate. For the purpose of removing this superfluous heat, a cooling plate 7 allowing a cooling medium to pass therethrough is disposed beside every cell laminate body 6 comprising a plurality of cells 4 which are laminated with a separator 5 being interposed between neighboring cells.
Further, if the fuel gas leaks outside the system, not only the utilization of fuel gas is deteriorated, but also there is a danger of explosion by the fuel gas. Therefore, a gas seal is applied, by making use of a sealing material 8, to a space between the solid polymer electrolyte membrane 3 and the gas-impermeable separator 5.
Additionally, at the location of the cathode 1b, water is generated due to an electrode reaction. When this water is condensed at the electrode reaction site, the diffusion of gas is badly affected. Therefore, the water thus produced is required to be discharged together with unreacted gas outside the cell.
On the other hand, as for the material for the solid polymer electrolyte membrane 3, a perfluorosulfonate film which is a fluorinated ion-exchange membrane is known. This solid polymer electrolyte membrane 3 contains an exchange group or hydrogen ion in its molecule and hence, functions as an ion conductive material when it is saturated with water.
However, once this solid polymer electrolyte membrane 3 is dried on the contrary, the ion conductivity thereof is lowered, thus prominently deteriorating the performance of cell. Therefore, there have been taken various measures to prevent the drying of the solid polymer electrolyte membrane 3.
For example, there is known a method wherein a humidifier constructed to allow water and a reactive gas to pass therethrough is disposed on both sides of a steam-permeating film such as the solid polymer electrolyte membrane 3 so as to allow the reactive gas to be wetted before it is fed to the solid polymer electrolyte membrane 3.
In this case, the humidifier is generally formed integral with the cell stack. Further, it is also known that if a reactive gas to be fed to the anode 1a and cathode 1b is allowed to flow to face each other and at the same time, the operation temperature is controlled to not more than 60xc2x0 C. so as to increase the relative humidity of the reactive gas, the generation of power can be achieved without necessitating to humidify the reactive gas.
On the other hand, as shown in Japanese Patent Unexamined Publication H6-132038, there has been also proposed a method of humidifying unreactive gas by introducing both reacted gas and unreacted gas into a gas chamber partitioned by means of a steam permeating film.
In this case, since water vapor is caused to generate on the cathode side 1b due to an electrode reaction, the reacted gas is rendered to contain saturated or nearly saturated water vapor.
On the other hand, since the quantity of water vapor contained in the unreacted gas is relatively small, a difference in partial pressure of water vapor is caused to generate between the reacted and unreacted gases, so that this difference in partial pressure of water vapor can be utilized as a driving force for effecting the concentration diffusion of water vapor.
Further, as shown in Japanese Patent Unexamined Publication H8-273687, there is also proposed to use a hollow fiber as a water vapor-permeating film, wherein unreacted gas is fed through the interior of the hollow fiber and the reacted gas is fed through the exterior of the hollow fiber, thereby humidifying the reactive gas.
Since the contact area between the reacted gas and the unreacted gas can be increased due to the employment of this hollow fiber, it becomes possible to provide a compact humidifier having a high humidification efficiency. Moreover, since a hollow fiber is employed, it becomes possible to incorporate the humidifier inside the gas manifold of the cell stack.
However, these conventional polymer electrolyte fuel cells systems as mentioned above are accompanied with various problems that when the reactive gas is to be humidified by means of a humidifier or through a humidity exchange, the resultant system becomes inevitably sophisticated and difficult to make it compact, and also may raise various problems when it is used in a low temperature environment such as an air atmosphere of 0xc2x0 C. or less.
In the case of the humidifier having a structure wherein water and reactive gas are allowed to flow along both sides of a water vapor-permeating film, the freezing of water passageway may be caused to generate when an external temperature is lowered, thus possibly inviting the closing of the passageway, the fracturing of the water vapor-permeating film due to the expansion in volume of ice, and the deformation of the separator 5.
On the other hand, if a non-humidifying operation is to be performed without employing a humidifier, it may become difficult to ensure the long term stability of the solid polymer electrolyte membrane 3 and of the cell performance. In addition to this problem, when the fuel cell is operated at a temperature of not more than 60xc2x0 C. which is lower than the ordinary operating temperature of 70 to 90xc2x0 C. by making use of a fuel gas containing CO as in the case of a reformed gas, the catalyst in the anode 1a is badly affected by this CO, thus resulting in the promotion of anodic polarization and hence, badly deteriorating the cell performance.
In the case of humidifying the reactive gas by respectively introducing reacted gas and unreacted gas into a gas chamber which is partitioned by means of a water vapor-permeating film, the transfer of water is effected only through a difference in partial pressure between these gases. Therefore, it has been impossible to obtain a sufficient degree of humidification because of a very large magnitude of diffusion resistance of water vapor such as the resistance to diffusion due to the concentration gradient of water vapor on the reacted gas side, the resistance to diffusion inside the water vapor-permeating film, and the resistance to diffusion on the unreacted gas side.
Further, there is also a problem as the cell stack and the humidifier are separately disposed that part of water vapor in the reacted gas is caused to condense in a midway of the tubing for introducing a reacted gas discharged from the cell stack into the humidifier, thereby decreasing the partial pressure of water vapor of the reacted gas to be fed to the humidifier, thus further deteriorating the humidifying efficiency.
When a hollow fiber is employed as a water vapor-permeating film on the other hand, since the transfer of water is effected only through a difference in partial pressure between the reacted gas and the unreacted gas, it has been impossible to obtain a sufficient degree of humidification though the contact area between the reacted gas and the unreacted gas can be increased. Further, if water vapor is condensed inside the hollow fiber to generate liquid water, the discharging of liquid water from the hollow fiber by means of a gas pressure would become difficult due to the capillary force, thus increasing the pressure loss of the gas and hence, deteriorating the efficiency of the system as a whole.
Therefore, an object of the present invention is to provide a polymer electrolyte fuel cells system which makes it possible to circulate water generated at an oxidizing electrode within the cell without sacrificing not only the property of cell but also the simplification and compactness of the system, thereby preventing a solid polymer electrolyte membrane from being dried, and which is excellent in performance and in compactness and capable of reliably actuating the cell system within a short time even if the ambient temperature is as low as not more than 0xc2x0 C.
With a view to achieve the aforementioned object, this invention provides a polymer electrolyte fuel cells system comprising a main cell body composed of a cell stack constituted by a laminated body of a plurality of cells each having a solid polymer electrolyte membrane, which is featured in that:
the main cell body is provided with temperature/humidity exchange means which enables a reacted gas passed through the cell stack to be contacted, through a water retentive-porous body, with an unreacted gas prior to a passage of the unreacted gas through the cell stack; and that
the temperature of the unreacted gas is controlled lower than the temperature of the reacted gas.
According to this invention as explained above, the unreacted gas of lower temperature is allowed to contact face-to-face with the reacted gas of higher temperature so as to produce the condensation of water vapor contained in the reacted gas in the interior of the water retentive-porous body, thereby allowing water to permeate close to an interface contacting with the unreacted gas due to an osmotic pressure of the interior of the porous body. Accordingly, a difference in partial pressure of water vapor at the interface contacting with the unreacted gas is caused to increase, thus making it possible to greatly decrease the diffusion resistance of water vapor on the occasion when the water vapor inside the reacted gas is transferred, via the porous body, to the unreacted gas through a temperature/humidity exchange.
According to this invention, there is further provided a polymer electrolyte fuel cells system comprising a main cell body composed of a cell stack constituted by a laminated body of a plurality of cells each having a solid polymer electrolyte membrane, the cells being laminated with a separator and a cooling plate being selectively interposed therebetween, which is featured in that:
the separator is provided with gas-feeding means which enables reactive gases from two lines to flow in a manner to face each other; and that
the cooling plate is provided with a coolant flow groove, the coolant upstream side of the flow groove being disposed at a gate portion of the reactive gases of the two lines in the separator.
According to this invention, there is further provided a polymer electrolyte fuel cells system comprising a main cell body composed of a cell stack constituted by a laminated body of a plurality of cells each having a solid polymer electrolyte membrane, the cells being laminated with a separator and a cooling plate being selectively interposed therebetween, which is featured in that:
the separator is provided with gas-feeding means which enables reactive gases from two lines to flow in a manner to face each other; and that
the cooling plate is provided with coolant flow grooves, intervals between which located at a gate portion of the reactive gases of the two lines in the separator are made narrower than intervals between the coolant flow grooves that are located other than the inlet or outlet portion of the reactive gases of the two lines in the separator.
According to this invention, there is further provided a polymer electrolyte fuel cells system comprising a main cell body composed of a cell stack constituted by a laminated body of a plurality of cells each having a solid polymer electrolyte membrane, the cells being laminated with a separator and a cooling plate being selectively interposed therebetween, which is featured in that:
the separator is provided with;
gas-feeding means which enables reactive gases from two lines to flow in a manner to face each other; and with
cooling means for radiating heat to outer atmosphere, the cooling means being located at a gate portion of the reactive gases of the two lines in the separator.
According to this invention as explained above, since the cooling efficiency becomes higher at a downstream portion of reactive gas where the partial pressure of water vapor is made higher due to water generated by the electrode reaction at the oxidizing electrode as well as due to the consumption of reactive gas, the downstream portion of the reactive gas becomes oversaturated with water vapor, thus causing the condensation of water to occur. The water thus condensed at the portion of the fuel electrode or the oxidizing electrode which is located at the downstream portion of the reactive gas is allowed to permeate into and evaporate from the upstream portion of the reactive gas at the oxidizing electrode or fuel electrode (both electrodes facing each other with a solid polymer film being interposed therebetween) where the partial pressure of water vapor is relatively low. As a result, the partial pressure of water vapor at the upstream portion of the reactive gas can be increased.
According to this invention, there is further provided a polymer electrolyte fuel cells system comprising a main cell body composed of a cell stack constituted by a laminated body comprising a plurality of cells each having a pair of gas diffusion electrodes consisting of a fuel electrode and an oxidizing electrode, and a solid polymer electrolyte membrane, the cells being laminated with a separator and a cooling plate being selectively interposed therebetween, which is featured in that:
the separator is provided with gas-feeding means which enables reactive gases from two lines to flow in a manner to face each other; and a downstream portion, in relative to reactive gas flow, of the gas diffusion electrodes is hydrophilized.
Although the gas diffusion layer is generally made hydrophobic for the purpose of preventing the occurrence of a gas diffusion failure due to water originating from a partial condensation of water vapor in the reactive gas, a downstream portion, in relative to reactive gas flow, of the gas diffusion electrodes is hydrophilized according to this invention as explained above so as to trap the condensed water by this downstream portion, in relative to reactive gas flow, of the gas diffusion electrodes without allowing the condensed water to be discharged. Namely, the water thus trapped by this downstream portion, in relative to reactive gas flow, of the fuel electrode or the oxidizing electrode is then allowed to permeate into an upstream portion of the reactive gas where the partial pressure of water vapor is relatively low in the fuel electrode or the oxidizing electrode, which are faced each other with a solid polymer electrolyte membrane being interposed therebetween, the permeated water being subsequently evaporated. As a result, the partial pressure of water vapor at the upstream portion of the reactive gas can be increased.
According to this invention, there is further provided a polymer electrolyte fuel cells system comprising a main cell body composed of a cell stack constituted by a laminated body comprising a plurality of cells each having a pair of gas diffusion electrodes consisting of a fuel electrode and an oxidizing electrode, and a solid polymer electrolyte membrane, the cells being laminated with a separator and a cooling plate being selectively interposed therebetween, which is featured in that:
the separator is provided with gas-feeding means which enables reactive gases from two lines to flow in a manner to face each other; and the entire passageway of reactive gas in the pair of gas diffusion electrodes excluding a downstream portion of the passageways is provided with a catalyst layer.
According to this invention as explained above, since the catalyst is not attached to a downstream portion of reactive gas where the partial pressure of water vapor is made higher due to water generated by the electrode reaction at the oxidizing electrode as well as due to the consumption of reactive gas, there is no possibility of generating a reaction heat originating from the electrode reaction at this downstream portion of reactive gas, thus enabling to maintain a low temperature of this downstream portion of reactive gas. As a result, the reactive gas can be cooled at the portions of electrodes which are located at this downstream portion of reactive gas, and hence, water vapor becomes oversaturated, thus causing the condensation of water. The water thus condensed at the portion of the fuel electrode or the oxidizing electrode which is located at the downstream portion of the reactive gas is allowed to permeate into and evaporate from the upstream portion of the reactive gas at the oxidizing electrode or fuel electrode (both electrodes facing each other with a solid polymer film being interposed therebetween) where the partial pressure of water vapor is relatively low. As a result, the partial pressure of water vapor at the upstream portion of the reactive gas can be increased.
According to this invention, there is further provided a polymer electrolyte fuel cells system comprising a main cell body composed of a cell stack constituted by a laminated body of a plurality of cells each having a solid polymer electrolyte membrane, which is featured in that:
the main cell body is provided with temperature/humidity exchange means which enables a reacted gas passed through the cell stack to be contacted, through a water retentive-porous body, with an unreacted gas to be passed through the cell stack; and with
means for condensing water vapor contained in the reacted gas, the means being attached to the porous body.
According to this invention as explained above, since water contained in the reacted gas can be removed, it is possible to prevent water from being kept remained in the discharge line of the reacted gas. At the same time, if the air inside a vehicle is employed as air of low temperature, the air can be utilized for heating the interior of the vehicle during a winter season, thus enhancing the efficiency of the system. Moreover, the condensed water can be utilized for the steam modification of a fuel modifier.
Therefore, it is now possible according to the present invention to circulate water generated at an oxidizing electrode within the cell without sacrificing not only the property of cell but also the simplification and compactness of the system, thereby preventing a solid polymer electrolyte membrane from being dried even if the quantity of water vapor in the reactive gas to be fed is small in a non-humidifying operation. Additionally, it is possible to provide a polymer electrolyte fuel cells system which is excellent in performance and in compactness and capable of reliably actuating the cell system within a short time even if the ambient temperature is as low as not more than 0xc2x0 C.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.